a) Field of the Invention:
The present invention relates to a zoom lens system which has high optical performance and is suited for use in video cameras, still video cameras and other optical appliances recently developed for picking up highly precise and minute images.
b) Description of the Prior Art:
Cameras which use solid-state image pickup devices generally adopt, as photographic lens systems, zoom lens systems having extremely high vari-focal ratios and images of relatively small sizes require optical systems which are bright and have high optical performance.
It is well known that such an optical system can be obtained by composing it of four lens units: in order from the object side, of a first lens unit having a positive power, a second lens unit which has a vari-focal function and a negative power, a third lens unit which serves for maintaining an image surface at a constant location during a change of a magnification and has a negative power, and fourth lens unit which has an imaging function and a positive power. However, such an optical system ordinarily has a large total length and can hardly meet the requirements recently imposed for configuring video cameras which are compact since an airspace must be reserved for moving the third lens unit, and the fourth lens unit is divided into a front subsystem which makes a diverging light bundle emerging from the third lens unit into a parallel light bundle and a rear sub-unit which images the light bundle.
In view of this fact, a zoom lens system is composed as a main trend these days, of four lens units, in order from the object side, a first lens unit having a positive power, a second lens unit which has a vari-focal function and a negative power, a third lens unit which has a positive power and is to be kept stationary during zooming, and a fourth lens unit which has a positive power and serves for correcting a deviation of an image surface caused by changing a magnification. Such a zoom lens system is configured to be focused by moving the fourth lens unit toward the object side.
Known as zoom lens systems of this type are conventional examples disclosed by Japanese Patents Kokai Publication Nos. Sho 62-178917, Sho 62-215225 and Sho 63123009.
Further, image pickup devices using picture elements having extremely small sizes are being developed for the purpose of offering compact image pickup devices usable for photographing the so-called highly precise and minute images such as those for displaying on high definition television. Since higher resolution is required for photographic lens systems as smaller picture elements are used for image pickup devices, the enhancement of optical performance of photographic lens systems is more strongly demanded.
Known as zoom lens systems satisfying such a demand are conventional examples disclosed by Japanese Patents Kokai Publication Nos. Sho 62-153913, Hei 1-126614 and Hei 6-56453.
It is general for obtaining high optical performance to properly cancel positive and negative aberrations produced by refracting surfaces with one another. In the case of a zoom lens system in which passage of rays and aberrations produced by refracting surfaces are largely different dependently on zoomed conditions, however, it is remarkably difficult to cancel aberrations produced by the refracting surfaces with one another at all focal lengths. When an attempt is made to shorten a total length of a zoom lens system which uses a zooming lens unit having a relatively simple composition, in particular, it is necessary to strengthen refractive powers of lens units, thereby allowing refracting surfaces to produce remarkable aberrations and making cancellation of aberrations much more difficult.
When the refractive power of the second lens unit is strengthened in an attempt to shorten the zoom lens system of the above-mentioned type (in which the four positive, negative, positive and positive lens units are disposed, and a negative refractive power is imparted only to the second lens unit having the vari-focal function, in particular) it is necessary to strengthen also the refractive power of the third lens unit or the fourth lens unit, thereby making it difficult to reserve a required back focal length or obtain high optical performance.
For obtaining a zoom lens system which has extremely high optical performance, high enough to accept highly precise and minute images, it is conceivable to reduce amounts of aberrations produced by refracting surfaces with an increased number of lens elements which refract rays at smaller angles and an increased number of times or cancel aberrations in a complicated way by selecting a complicated composition for a zooming lens unit. However, either of these conceptions has a defect resulting in the enlargement of the zoom lens system.
Each of the above-mentioned conventional examples disclosed by Japanese Patents Kokai Publication Nos. Sho 62-178917, Sho 62-215225 and Sho 63-123009 has a relatively simple composition and is remarkably compact, but exhibits optical performance insufficient as that of a photographic lens system for accepting the highly precise minute images such as those for high definition television. Further, each of the conventional examples disclosed by Japanese Patent Kokai Publication Nos. Sho 62-153913, Hei 1-126614 and Hei 6-56453 has extremely high optical performance, but cannot be said to be a sufficiently compact zoom lens system since it uses a large number of lens elements and is configured to cancel aberrations in a complicated way by using a zooming lens unit having a complicated composition.
For obtaining a compact zoom lens system which has high optical performance it is therefore desirable to select a zoom type advantageous for compact design of a lens system and contrive compositions of lens units without excessively strengthening refractive powers thereof. Speaking concretely, it is important to contrive, while selecting the composition of the above-mentioned zoom lens system composed of the four positive, negative, positive and positive lens units and contriving compositions of the third lens unit and the fourth lens unit without remarkably strengthening the refractive power of the second lens unit.